The present invention relates to a method for controlling the removal, by means of ion etching, of a thin layer, or regions of a layer as determined by masks, from a substrate which has a different chemical composition than that of the layer to be removed. The invention further relates to a suitable apparatus for practicing this method.
In the process of ion etching, the effect of cathode sputtering is utilized for the directed removal of surface layers, i.e., the layer regions to be removed are sputtered by ion bombardment and are thus removed. The ions may be generated, for example, in an ion source and then accelerated in the direction of the sample. Ion etching for the directed removal of surfaces (ion polishing and ion milling) produces a depth resolution in the atomic region and is used, inter alia, for the removal of surface adhering impurities. For example, an oxide film of a typical thickness of 100 Angstroms can be removed from a sample with minimum stress and destruction of The sample by an ion etching process.
The periodical "Vakuumtechnik", Volume 24, Issue No. 6, September 1975, describes the removal by ion etching of certain layer regions which are defined by masks on a layer which is applied to a substrate and whose thickness lies, for example, in the micron range. The substrate may also be a second layer of material of a multi-layer sample which can be further removed in sections by means of a further etching process in the regions in which the upper layer has already been removed. The use of this etching technique is particularly appropriate in the manufacture of microelectronic components.
In most cases of use of the ion etching process there results the problems of being able to accurately control the ion etching process, particularly the etching depth, in order, for example, to terminate the ion etching process when a certain boundary layer of a multi-layer system has been reached. If, for example, a resistor or conductor structure in an integrated circuit is to be produced by means of ion etching, it may be desirable to terminate the ion etching process precisely when a clean substrate surface has been reached in order to prevent damage to the substrate or the remaining surface structures. Since any type of material may be removed in the ion etching process there often arises the problem of stability of the masks. Since the masks are removed particularly easily along their edges if the ion bombardment takes place at an acute angle, there exists the danger that the masks will shrink if the ion etching process continues for too long a period of time so that the desired structure can no longer be obtained.
The precise determination of the etching depth has been difficult to accomplish in the prior art ion etching systems. Optical controls are usually insufficient or even often impossible due to the structure of the apparatus. Analysis processes, for example with the aid of electron spectroscopy or mass spectroscopy, are complicated and expensive and are generally combined with an ion etching system only in research plants.